PkthybridgI'; and Mttkphy — Baclerial Discnse of the. Fotatn. 27 



(29) Sterile Slices of cooked Potato. — At 25° C. a yellowish, slim}-, shiuing 

 growth occurs in two days, which gradually spreads over the surface without 

 being raised above it to any extent. This is the only medium on which we 

 have found the organism to exhibit any chromogenic character. After six days 

 the portions of the potato-slices not covered by bacterial growth become 

 brownish ; control, non-inoculated slices remain uncoloured. The potato-slices 

 became alkaline, controls remaining acid. 



(30) Slices of living Potato aseptically prepared. — At room-temperature a 

 slight brown discolouration and a shght sinking-in of the tissues occur within 

 twenty-four hours. By the third day the depression is well marked, the 

 l^ottom and sides of it being of a lightish brown colour, and the margin a deeper 

 1 irown or almost black. On standing longer the depression frequently becomes 

 somewhat deeper and more or less blackened all over. Fig. 7, Plate III., 

 shows the results of four days' growth of the organism on living potato-slices. 

 The tissues become soft and pulpy to the bottom of the slice if about half an 

 inch thick. Microscopic examination of the pulp shows that the middle 

 lamellae of the cells have become dissolved, lea^dug the cells themselves free 

 and with shghtly browned protoplasmic contents, but with intact starch-grains 

 in which the lines of stratification are often very well marked. The inception 

 and progress of the attack on raw potato-slices in Petri dishes are largely 

 dependent on the presence of moisture. Success is best attained when care is 

 taken to keep the interior of the Petri dish well moistened. 



(31) Whole Potatoes inoculated through the " heel." — The method used has 

 already been described on p. 17 . At room-temperature, within forty-eight 

 hours a sUght, slimy exudation can be observed, together with a blackening 

 around the edge of the wound. This blackening frequently spreads centri- 

 fugally over the skin, being marked off by a fauiy sharp Hne from the normal 

 skin. The rot extends beneath this discoloured skin ; and if lenticels be 

 present on this area, they often stand out sharply as black spots. When 

 affected or artificially inoculated tubers are of a red variety — that is, when the 

 cells of a few layers of the tissue immediately below the true cork cells of 

 the skin contain anthocyan in solution in the cell-sap — the colourmg-matter 

 becomes destroyed in the attacked portions, and the surface of the tuber no 

 longer shows the colour characteristic of the healthy tuber over these areas. 

 When cut longitudinally through the inocxilation-stab, the cavity is more or 

 less filled with a soft, pulpy mass usually of a light brown colour. This pulp 

 is frequently marked off from the still unattacked tissues by a distinct black 

 line. In some cases the organisms pass into the " vascular ring," and cause it 

 to become brown for a greater or less portion of its circiunference. (See fig. 5, 

 Plate II.) In some cases the pulpy materialis almost pure white ; and the dark 



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